Basically, an indexing head is a device that holds a part firmly in place in order that a machining operation (such as drilling a hole) can be accomplished. A spindle within the head can be rotated to a new radial position and another operation performed, thus called an indexing head. While this idea basically has been around since the beginning of machine shops, there have been many devices built and used to accomplish the task. They vary in size, the method of holding the work piece, and the means of rotating the work.
The basic spindle and casting of a standard indexing head was originally designed by Charles Tree and patented March, 1941, U.S. Pat. No. 2,233,858. His device describes a body that houses a rotating spindle. The inside of the spindle has been conformed to accept a collet. A collet can be thought of as three fingers grabbing the outside of a round part. These fingers are then drawn into a taper inside the spindle causing the fingers to tighten round the work, holding it firmly so that work can be performed. A flat plate was then fixed to the rear of the spindle. This plate contained a number of holes (usually 20 or 24) and was designed so that a pin located in the body could drop into the holes in the plate and lock the spindle in place. By lifting the pin out of the hole in the plate the spindle could then be rotated to another position and the pin reinserted to lock the spindle. A very useful idea incorporated into the plate was the ability to lock out any number of holes so that the pin could drop into only the holes desired. The head tree designed was small in nature and easy to use. The head was so successful that one can be found in almost every machine shop today.
A patent was granted to Robert Path in 1972, U.S. Pat. No. 3,661,403, for an improvement to the indexing fixture of Charles Tree. Path's idea was to overcome the limitations of 20 or 24 position indexing by designing an indexing pin that used a multi-tooth serated pin that fit into a corresponding gear to give a higher number of indexing positions. The serations were designed to give 360 individual stop positions or 1 degree divisions. The major flaw to this design was that there was no way of blocking unnecessary positions. The operator would have to locate each desired stop visually before allowing the pin to lock into the plate resulting in an incredibly slow and tedious operation.